The invention of the present application relates to a fluid pressure control device for actuating an external object. The control device of the present application is particularly useful in the backwashable filter described in patent application Ser. No. 6599,261, now U.S. Pat. No. 4,614,581, for initiating a backwash operation when the pressure drop across the filter body rises above a predetermined magnitude, thereby indicating that the filter body is overly clogged and should be cleaned.